Recently, as the demand for mobile products such as a mobile phone terminal and a personal digital assistant (PDA) rapidly increases, a variety of attempts to reduce the current consumption of a semiconductor memory device mounted in such mobile products are continuously being made. In particular, to reduce a refresh current of a semiconductor memory device for mobile products is becoming a large issue.
Among semiconductor memory devices, Dynamic Random Access Memory (DRAM) stores information in memory cells by charging capacitors of the memory cells, different from Static Random Access Memory (SRAM) or flash memory. Since capacitors of the DRAM leak charge, the information eventually fades unless the capacitor charge is refreshed periodically. Such an operation is referred to as a refresh operation. The refresh operation is performed by activating memory cell arrays included in the semiconductor memory device at least one time within a retention time and amplifying data stored therein. Here, the retention time refers to a time during which data can be maintained without a refresh operation after the data are recorded in memory cells.
In general, a semiconductor memory device includes a plurality of banks, and memory cell arrays included in each of the banks are grouped as a plurality of mats. For example, if a 512M semiconductor memory device includes eight banks, each of the banks includes 64M memory cell arrays. Also, if memory cell arrays included in each of the banks are grouped as 32 mats, each of the mats includes 2M memory cell arrays. Further, for example, each of the mats includes 512 row lines. Specifically, each of the mats includes 64 main word lines, each of which has eight sub-word lines.
A refresh operation of a semiconductor memory device may be performed by activating the mats and simultaneously activating specific main word lines and specific sub word lines included in the activated mats. That is, the refresh operation may be performed by activating all main word lines and all sub word lines according to a sequence in which first main word lines and first sub word lines of all the mats are activated, and second main word lines and second sub word lines of all the mats are then activated, for example.
In the refresh operation performed in such a manner, all the mats are sequentially activated in a state in which the addresses of the main word lines and the sub word lines to be activated are fixed. Therefore, until counting all bits of row addresses for activating mats is terminated, row addresses for activating the main word lines and the sub word lines are not counted. Accordingly, a constant value is maintained.
However, row addresses are latched in synchronization with a pulse signal toggling during a refresh operation period. Therefore, even in a state in which the row addresses for activating the main word lines and the sub word lines are not counted and thus a constant value is maintained, the row addresses are continuously latched in synchronization with the pulse signal. Therefore, power consumption occurs.